It is well-known to provide a brake system for a bicycle. Such known brake systems comprise a brake mechanism at a wheel (or at each wheel) and a brake actuation mechanism that allows the rider to apply braking force selectively to maintain control while riding the bicycle. It is known to provide a brake system with a brake actuation mechanism in the form of a brake master cylinder assembly that is mounted on the handle bar of the bicycle and actuated by hand by the rider. Such known assemblies comprise a hand lever for the rider and are configured to transmit action applied by the rider at the hand lever to the brake mechanism at a corresponding wheel of the bicycle.